G-7500 Equalizer / Pre-Amp Fix?

[gregmcm]

Picked up a G-7500 project with mostly cosmetic issue. Cleaned it up, deoxit the switches and turned it on. Lights up, no smoke and nothing unusual . Checked AM, FM, Aux and all working fine. Didn't immediately try the phono inputs on the assumption they would be fine but now that I've plugged in a turntable I discover no sound on either Phono 1 or 2. Bad assumption. Checked turntable and it is in working order. Also did another deoxit of the selector switches as they are always problematic on these old units but still nothing out of the phono inputs.

My thinking is that the F-2852 Equalizer Circuit Board is where the problem likely resides. A visual inspection and there are no obvious signs of component failure. At this point I'm thinking a transistor problem. There are 8 transistors and 4 FETS on the board.

Questions:
- should I check the transistors or the FETS first?
- can these be checked in circuit?
- is going down the road of checking the transistors/Fets the right track or is there some other test that would be better as a starting point?

I have the service manual, schematic and a digital multi meter. Thanks

 

[Hyperion]

Questions:
- should I check the transistors or the FETS first?

No, see #3 - Although it could be that the phono inputs have been overdriven by someone, and this damaged the input device(s). Removing and potentially damaging unobtanium input devices to test them, wouldn't be first on my list.

- can these be checked in circuit?

You could, but you would be measuring voltages, and voltage drops across components to establish the likelihood of correct current flow. (check the 30V supply to the EQ PCB on both sides of R46 (x2)

- is going down the road of checking the transistors/Fets the right track or is there some other test that would be better as a starting point?

You need to verify the switching path to the phono inputs, with either a multimeter, an Oscilloscope +Signal generator or Signal Tracer. And ensure all such switching paths are clean (DeOxit)

 

[gregmcm]

John, thanks -- I like your number 3 suggestion. It will take me a bit to figure out the signal path given I'm an amatuer at this but I'm determined and there's no hurry.

 

[Hyperion]

Seems to be only S601a & S601b on the input side and S601c & S601d on the output side as it turns out. (providing everything else works).

I have updated my first post with another check.

 

[gregmcm]

John, excuse my ignornance but having trouble identifying the 601 locations -- can you tell me what board these are on? Here is a portion of the schematic for the Equalizer Board.

 

[kevzep]

I had a G7000 come in with the phono not working, turned out to be a VD1212 diode had given up....Looks like there is the MV12 (x 2) in this one, would be worth checking, but since both channels aren't working, it could be a power supply issue so that's where I could start.

 

[Hyperion]

John, excuse my ignornance but having trouble identifying the 601 locations -- can you tell me what board these are on? Here is a portion of the schematic for the Equalizer Board.

The 5 position switches on the input and output of the EQ. S601a,c for the top section and S601b,d for the bottom.

With reference to Kev's comments:-

(check the 30V supply to the EQ PCB on both sides of R46 (x2)

 

[gregmcm]

The 5 position switches on the input and output of the EQ. S601a,c for the top section and S601b,d for the bottom.

With reference to Kev's comments:-

John, I'm catching on so bear with me if you will. What I'm not clear on is how to identify the input output signal path on the 5 position switch. I see where each section of the switch (there are three sections) appears to have 5 contacts but what I can't identify is what each contact of each switch is identified as -- I'm assuming they are identified as a,b,c,d,e but that's a guess on my part at this point in my understanding of how this switch works. Any insight into how the contacts for each switch are identified would be great. For example, and assuming I'm on the right path, would they be a,b,c,d,e in a clockwise direction looking at the receiver from the front?

Note: the switch contacts are clearly oxidized and mostly black. I verified this using a q-tip and some deoxit on a portion of the rear switch that I could access with the q-tip. Looks like the rear switch is more black than the middle or front switch but all are for sure black.

 

[gregmcm]

I had a G7000 come in with the phono not working, turned out to be a VD1212 diode had given up....Looks like there is the MV12 (x 2) in this one, would be worth checking, but since both channels aren't working, it could be a power supply issue so that's where I could start.

kev, checked the diodes and they are fine. With regard to checking the power supply can you tell me where I need to connect the DMM (+ and -) to in order to check this?

 

[gregmcm]

I had a G7000 come in with the phono not working, turned out to be a VD1212 diode had given up....Looks like there is the MV12 (x 2) in this one, would be worth checking, but since both channels aren't working, it could be a power supply issue so that's where I could start.

KEV, went ahead and checked the DCV across the r46 resistor and got a reading of 32.6. Can I assume the power supply isn't the issue?

 

[Hyperion]

Some terminology might be confusing us all here, If you checked 'across' the resistor (this means one probe at each end of the resistor R46) - this indicates you have a short circuit on the EQ side of the resistor, because all of the voltage applied at one end of the resistor is being dropped 'across' this resistor. This doesn't seem likely, as that would mean the little R46 resistor was dissipating 10W - but, it's important to get this principle understood - most measurements will be from a point to ground, but some will be 'across' a component - there is an important distinction.

If you measured with one probe at either end of the resistor, and the other probe to ground (as I suspect you did actually) - then that is ok - can you clarify how you measured this voltage?

appears to have 5 contacts but what I can't identify is what each contact of each switch is identified as -- I'm assuming they are identified as a,b,c,d,e but that's a guess on my part at this point in my understanding of how this switch works

Some clarification, it is the switch sections or 'wafers' or 'sections' that are labelled with letters - so in this case you will have 5 (fixed) contacts connecting to one 'wiper' (moving) contact this is a 'section' and might be called 'a' - in this case S601a. There may be 1 or more sections to a 'wafer' maybe 2 sections in this case (?) in which case there could be 2 sets of 5 fixed contacts and 2 'wiper' contacts - which would be called S601a,b perhaps (for one channel) - and in this case you will likely have a separate wafer with the same arrangement for S601c,d. (for other channel) - does that make it any clearer?

 

[Pepa]

Would like to add....R45,46,47 and 48 are FR

 

[gregmcm]

Some terminology might be confusing us all here, If you checked 'across' the resistor (this means one probe at each end of the resistor R46) - this indicates you have a short circuit on the EQ side of the resistor, because all of the voltage applied at one end of the resistor is being dropped 'across' this resistor. This doesn't seem likely, as that would mean the little R46 resistor was dissipating 10W - but, it's important to get this principle understood - most measurements will be from a point to ground, but some will be 'across' a component - there is an important distinction.

If you measured with one probe at either end of the resistor, and the other probe to ground (as I suspect you did actually) - then that is ok - can you clarify how you measured this voltage?

- here are my findings when testing the R46. One probe at each end of the resistor and the meter reads 32.6V. If I test grounding the black probe lead and touching the red to R46 I get 32.6V on one side of the resistor and -1.4V on the other side.


Some clarification, it is the switch sections or 'wafers' or 'sections' that are labelled with letters - so in this case you will have 5 (fixed) contacts connecting to one 'wiper' (moving) contact this is a 'section' and might be called 'a' - in this case S601a. There may be 1 or more sections to a 'wafer' maybe 2 sections in this case (?) in which case there could be 2 sets of 5 fixed contacts and 2 'wiper' contacts - which would be called S601a,b perhaps (for one channel) - and in this case you will likely have a separate wafer with the same arrangement for S601c,d. (for other channel) - does that make it any clearer?

Getting clearer for sure -- will work on sorting out how to test if the switch is performing correctly this morning. Here is a photo of the switch and the EQ board.

 

[gregmcm]

In case this helps here is a better quality scan of the F-2852 Equalization Board.

 

[Hyperion]

Seems to be 3 groups of pins (wafers) from the switch, it would appear that the rearmost wafer deals (2 sections?) with the input switching and the middle wafer deals with the output switching (2 sections). The front wafer seems to switch connections that come to the EQ board via connectors - through the switch and out again via more connectors. (I am assuming the right is towards the front panel and where the switch spindle is).

Can you see how I deduced that?

NB: If you clean the switch properly you might solve the problem at a stroke, you mention 'black contacts'.

All this seems very laboured - but this is electronics testing and important to understand the process for eliminating faults - divide and conquer. I could send you straight to checking voltages at the transistors, or injecting a signal and tracing where it gets to before it is lost, this is what an experienced tech would be able to do in a matter of minutes, but far too difficult for most who are new to this.

 

[gregmcm]

Seems to be 3 groups of pins (wafers) from the switch, it would appear that the rearmost wafer deals (2 sections?) with the input switching and the middle wafer deals with the output switching (2 sections). The front wafer seems to switch connections that come to the EQ board via connectors - through the switch and out again via more connectors. (I am assuming the right is towards the front panel and where the switch spindle is).

Can you see how I deduced that?

All this seems very laboured - but this is electronics testing and important to understand the process for eliminating faults - divide and conquer. I could send you straight to checking voltages at the transistors, or injecting a signal and tracing where it gets to before it is lost, this is what an experienced tech would be able to do in a matter of minutes, but far too difficult for most who are new to this.

John, yes - this is making sense to me and I believe I'm on the right track as what I am seeing with the rear and middle wafer appear to be deal with the switching from the the phono input to the output. Using the DMM I'm working on checking if the wafer switch is doing it's job and transferring the input signal to the outputs.

With regard to the resistors at R45,46,47,48 this is what I found. Ground to one side of each shows: R45 - 29.9V, R46 - 32V, R47 - 14.6V and R48 - 32.8V

 

[Hyperion]

That R47 -14.6V is a problem - either the resistor has gone high in value, OR C37 (220µF, 35V) is very leaky or installed the wrong way round.

OR there is a problem further downstream (on the EQ PCB) where too much current is being pulled.

 

[gregmcm]

There are no obvious signs that the C37 capacitor is leaking and it is installed correctly. Also looks like the wafer switch is doing it's job. I'm going to pull the board as this is necessary to gain access to the conductor side and remove the two suspect components to check them out of circuit and depending on what I find replace them or continue my quest.

 

[gregmcm]

OK -- here is what my testing out of circuit found:
Capacitors:
C35 - 243mf
C36 - 245mf
C37 - 184mf
C38 - 250mf
Going to replace these and the other caps on this board since I'm in there anyway.

Resistors:
Two test at about 90 resistance and two show nothing on the DMM. Going to replace all 4.

Heading out to the parts store to pick up some parts. Have my fingers crossed.

 

[gregmcm]

SUCCESS! Replace the caps on the EQ board as well as the four resistors and now have a phono stage which is very gratifying. Thanks for all the help. I'll give it a good workout and hopefully all will be well for the next number of years.